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Dell PowerVault ME5 Series Administrator's Guide

ADAPT reconstruction

Reconstruction of an ADAPT disk group is similar to reconstruction of a RAID-6 disk group, and can be impacted by host I/O activity and other processes running on the storage system.

ADAPT reconstruction differs from reconstruction of a RAID-6 disk group as follows:

  • When one disk is failed, not all stripes will be degraded: there will be a mix of fault tolerant and degraded stripes.
  • When two disks are failed, not all stripes will be critical: there will be a mix of fault tolerant, degraded, and critical stripes.
  • Reconstruction will generally complete more quickly than for RAID-6.
  • Reconstruction will start immediately without waiting for replacement of the failed disk.
    NOTE If a disk fails in an ADAPT disk group and is replaced by a new disk in the same slot as the failed disk, the disk group automatically incorporates the replacement disk into the disk group.
  • Reconstruction will start on spare capacity already available in the ADAPT disk group.
  • When there are critical stripes (and enough spare space), there will be two separate reconstruction phases: a first phase to repair critical stripes (to degraded state) and a second phase to repair the degraded stripes. Each phase will have its own start and end events. Because of the two-phase rebuild, ADAPT might take longer to reconstruct to fault-tolerant state than a critical RAID-6 running two-disk reconstruct. However, the first phase reconstruction of ADAPT, from critical state to degraded state, will be much faster. You can monitor reconstruction and rebalancing progress from the Activity panel.

If the ADAPT disk group has no spare space, the REFT (rebalance fault tolerant stripes) utility will run. As spare space is completely used, some stripes are critical, some are fault tolerant, and most are degraded. This utility attempts to rebalance stripe health away from the critical state and towards the degraded state. Stripes that are fault tolerant give up one of their disks, making them degraded. This disk capacity is then used to make a critical stripe zone degraded. It is recommended that spare space is added to the pool by either replacing failed disks or expanding the ADAPT disk group, and never to let spare space run out. However, if spare space is lost, the REFT utility attempts to give the ADAPT disk group the best redundancy across the whole disk group.

NOTE Rebalancing-applicable only to ADAPT-will commence on the newly replaced disk. Use cases for rebalancing are described below:
  • If the failed disk is replaced immediately, such that all stripe zones are fault tolerant, then only rebalancing occurs.
  • If the failed disk is replaced later, and more disks have failed (such that there is limited or no spare space), then multiple stripe zones have likely become degraded or critical. Reconstruction will be followed by rebalancing.
  • If no default spare space was selected, then reconstruction will occur without subsequent rebalancing

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